This invention relates to bisphenol-A/terephthalate/carbonate copolymers characterized by having a ratio from 2:0.8:1.2 to 2:1.3:0.7 of bisphenol-A ("BPA"):terephthalic acid ("TPA"):carbonate moieties combined in the polymer product; by having short average lengths of the polyester and polycarbonate segments; by having a high degree of freedom from the nitrogen base and/or organic acid impurities inherent in prior bisphenol-A/terephthalate/carbonate copolymers, such that the polymers of the invention are melt processable; and having properties making our polymers uniquely useful.
Especially they are useful for glazing and transparent sheet applications demanding high impact resistance, high resistance to scratching and abrasion, and/or high solvent resistance. They exhibit melt processability, high tensile strength, high impact strength, and good clarity and freedom from color; approximating in these qualities the known commercial polycarbonates. They are unique in combining with the foregoing, as such and without being coated or modified, high resistance to abrasion and scratching; high resistance when under stress to the action of many common solvents including in particular hot water, carbon tetrachloride, toluene, gasoline, butyl acetate, and acetone; high dimensional stability at elevated temperatures as reflected in the high values for glass transition temperature ("T.sub.G ") and relatively small difference between T.sub.G and heat distortion temperature ("HDT"), and in low values for creep (i.e. extension under load over an extended time period) at elevated temperature; and high stability against heat as shown by retention of molecular weight upon heat treatments and low weight loss upon thermal gravimetric analysis ("TGA").
Polyester/carbonate copolymers have been the subject of prior investigations, including such copolymers from BPA reacted with carbonate precursors and with dibasic acids, especially adipic acid and isophthalic acid. A few tests using terephthalic acid have been reported (U.S. Pat. Nos. 3,030,311 of Apr. 17, 1962 and 3,169,121 of Feb. 9, 1965 to Eugene P. Goldberg; and a corresponding literature article in Polymer Preprints, vol. 5, No. 1 of 1964, pp. 233-238). The disclosure of U.S. Pat. No. 3,169,121 is closely similar to that of U.S. Pat. No. 3,030,331; the discussion below refers to U.S. Pat. No. 3,169,121 unless otherwise noted.
Two tests are reported using as reactants bisphenol-A (hereinafter "BPA"), terephthalic acid (hereinafter "TPA") and phosgene. One was at reactant mol ratio of 2:1 of BPA:TPA in a stirred pyridine medium, into which phosgene was bubbled until the reaction mixture became viscous (see col. 6, lines 47-54 and Ex. 5). The other (Ex. 12) was the same except using half as much TPA, i.e. 2:0.5 mol ratio of BPA:TPA.
The only properties given for the resulting polymers are Intrinsic Viscosity, Softening Temperature, and Tensiles (Strengths and Elongations). By contrast, there are three examples using isophthalic acid (Exs. 3, 10, 11) each of which reports the following additional properties: Transfer Molding Temperature, Heat Distortion, Impact Strength, Wt. Loss (percent at 24 hours, 230.degree. C.), Flexural Strength, and Stiffness. It is explained in the literature article (page 238) that the tensile testing was on solution-cast film and that the impact and "T.sub.G " tests were on transfer-molded bars. (The "T.sub.G" figures in Table 1 of the literature article are the same figures as "Heat Distortion" in the patent examples and were determined by a penetration method also appropriate for heat distortion measurements; for methods of measurement, the literature article cites a Goldberg paper "J. Polymer Sci., in press" which article appeared in J. Polymer Sci., Part C, No. 4 of 1964, pages 707-730, see pages 715-717).
We have found that the BPA/TPA/carbonate copolymers prepared in pyridine reaction medium as prescribed by Goldberg:
(1) Do not correspond in composition to the ratios of the BPA:TPA reactants in the feed, but instead have markedly lower contents of TP moieties; and
(2) Are thermally unstable under molding conditions, to the extent that moldings therefrom show bubbles and discolorations and show serious deterioration in viscosity number ("I.V.") of their solutions compared to that before molding. Correspondingly, these polymers prepared in pyridine reaction medium have Izod impact resistance of only about 1 to 2 ft.-lb. per inch of notch, measured by the usual method, compared to values of 12-16 ft.-lb. per inch for commercial polycarbonates. This deterioration under molding conditions is believed to explain why the Goldberg patents and literature article are silent as to properties such as molding temperature, impact resistance, heat distortion temperature and glass transition temperature (T.sub.G) (as determined by penetration method) of his BPA/TPA/carbonate copolymers: those properties which require exposure to high temperatures in a molding operation, for purposes of measurement, are not reported by Goldberg for his BPA/TPA/carbonate copolymers.
The above phenomena of low TP moiety content in the polymer and deterioration of such polymer under molding conditions, we believe from our investigations to be due to some sort of interaction between terephthalic acid and pyridine, whereby the polymer, prepared in straight pyridine reaction medium, contains residues of pyridine in amounts on the order of 1%, strongly bound in the polymer. Also we find that TPA has to some extent modified the Goldberg polymer structure, in the form of terephthalic acid anhydride linkages incorporated in the chain.
When terephthaloyl chloride ("TPC") is used in pyridine reaction medium, instead of TPA, in accordance with one suggestion of the Goldberg patents, the resulting polymer still shows the above deficiencies.
Intensive purification procedures of such prior polymer lead to property improvement, e.g. impact resistance; but the polymer product following Goldberg still retains significant quantities of pyridine and acid impurities, still shows poor color after molding, still changes greatly in I.V. after heating and still falls well short of having as high a content of the TP moiety in the polymer as in the reactant mixture. In fact, we have found, the use of Goldberg's pyridine reaction medium precludes obtaining a ratio of BPA:TP moieties in the polymer product any higher than 2:0.8 even when the TPA or TPC ingredient is used in large excess over a 2:1 mol ratio.